It is known to perform a hydroprocessing step, e.g. hydrocracking and/or hydrofinishing, in the manufacture of base oils. The disadvantage of such hydroprocessed oils, in particular hydrocracked base oils, is that they are unstable to daylight, i.e. to short wave-length light characteristic of daylight. Exposure of hydroprocessed base oils to daylight usually produces one or more of the following undesirable effects: darkening, haze formation and sludge formation. Attempts have been made to overcome this disadvantage by blending the base oil with various other oils, for instance, non-hydroprocessed oils, but such other oils have to be used in quite large amounts, e.g., in excess of 5 %w, which can influence the properties of the base oils. Attempts have also been made to overcome this disadvantage by including additives in the hydroprocessed base oils. For example, U.S. Pat. No. 3,702,299 discloses the use of orthohydroxybenzophenones and aliphatic primary amines and U.S. Pat. No. 3,756,943 discloses the use of hindered phenols, amines and Mannich condensation products from phenols, aldehydes and polyamines.